关于java:Map根据值获取key

Map 依据值获取 key，找到一篇大佬的文章，本文是 CV 过去的，
原文：https://www.techiedelight.com…

1.entrySet() 办法

import java.util.HashMap;
import java.util.Map;
 
class Main
{public static <K, V> K getKey(Map<K, V> map, V value)
    {for (Map.Entry<K, V> entry: map.entrySet())
        {if (value.equals(entry.getValue())) {return entry.getKey();
            }
        }
        return null;
    }

    // Java8
    public static <K, V> K getKey(Map<K, V> map, V value)
    {return map.entrySet().stream()
                .filter(entry -> value.equals(entry.getValue()))
                .findFirst().map(Map.Entry::getKey)
                .orElse(null);
    }
 
    public static void main(String[] args)
    {Map<String, Integer> hashMap = new HashMap();
        hashMap.put("A", 1);
        hashMap.put("B", 2);
        hashMap.put("C", 3);
 
        System.out.println(getKey(hashMap, 2));        // prints `B`
    }
}

2.keySet() 办法

import java.util.HashMap;
import java.util.Map;
 
class Main
{public static <K, V> K getKey(Map<K, V> map, V value)
    {for (K key: map.keySet())
        {if (value.equals(map.get(key))) {return key;}
        }
        return null;
    }

    // Java8
    public static <K, V> K getKey(Map<K, V> map, V value)
    {return map.keySet()
                    .stream()
                    .filter(key -> value.equals(map.get(key)))
                    .findFirst().get();
    }
 
    public static void main(String[] args)
    {Map<String, Integer> hashMap = new HashMap();
        hashMap.put("A", 1);
        hashMap.put("B", 2);
        hashMap.put("C", 3);
 
        System.out.println(getKey(hashMap, 2));        // prints `B`
    }
}

3. 反转 Map

import java.util.HashMap;
import java.util.Map;
 
class MyHashMap<K, V> extends HashMap<K, V>
{Map<V, K> reverseMap = new HashMap<>();
 
    @Override
    public V put(K key, V value)
    {reverseMap.put(value, key);
        return super.put(key, value);
    }
 
    public K getKey(V value) {return reverseMap.get(value);
    }
 
}
 
class Main
{public static void main(String[] args)
    {MyHashMap<String, Integer> hashMap = new MyHashMap();
        hashMap.put("A", 1);
        hashMap.put("B", 2);
        hashMap.put("C", 3);
 
        System.out.println(hashMap.getKey(2));        // prints `B`
    }
}

4.Guava’s BiMap 类

import com.google.common.collect.BiMap;
import com.google.common.collect.ImmutableBiMap;
 
class Main
{public static <K, V> K getKey(BiMap<K, V> map, V value) {return map.inverse().get(value);
    }
 
    public static void main(String[] args)
    {BiMap<String, Integer> bimap = ImmutableBiMap.of("A", 1, "B", 2, "C", 3);
 
        System.out.println(getKey(bimap, 2));        // prints `B`
    }
}

5.Apache Commons Collections

import org.apache.commons.collections4.BidiMap;
import org.apache.commons.collections4.bidimap.DualHashBidiMap;
 
class Main
{public static <K, V> K getKey(BidiMap<K, V> map, V value) {return map.inverseBidiMap().get(value);
    }
 
    public static void main(String[] args)
    {BidiMap<String, Integer> bimap = new DualHashBidiMap<>();
        bimap.put("A", 1);
        bimap.put("B", 2);
        bimap.put("C", 3);
 
        System.out.println(getKey(bimap, 2));        // prints `B`
    }
}

原文：https://www.techiedelight.com…